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# Density

For other meanings of density, see density (disambiguation)

Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. The higher an object's density, the higher its mass per volume. The average density of an object equals its total mass divided by its total volume. A denser object (such as iron) will have less volume than an equal mass of some less dense substance (such as water).

The SI unit of density is the kilogram per cubic metre (kg/m3)

$\rho = \frac{m}{V}$

where

ρ is the object's density (measured in kilograms per cubic metre)
m is the object's total mass (measured in kilograms)
V is the object's total volume (measured in cubic metres)
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## Various types of density

Under specified conditions of temperature and pressure, density of a fluid is defined as described above. However, the density of a solid material can be different, depending on exactly how it is defined. Take sand for example. If you gently fill a container with sand, and divide the mass of sand by the container volume you get a value termed loose bulk density. If you took this same container and tapped on it repeatedly, allowing the sand to settle and pack together, and then calculate the results, you get a value termed tapped or packed bulk density. Tapped bulk density is always greater than or equal to loose bulk density. In both types of bulk density, some of the volume is taken up by the spaces between the grains of sand. If you are interested in the density of the grain of sand itself you need to measure either envelope density or absolute density.

## Other units

Density in terms of the SI base units is expressed in terms of kilograms per cubic metre (kg/m³). Other units fully within the SI include grams per cubic centimetre (g/cm³) and megagrams per cubic metre (Mg/m³). Since both the litre and the tonne or metric ton are also acceptable for use with the SI, a wide variety of units such as kilograms per litre (kg/L) are also used. Imperial units or U.S. customary units, the units of density include pounds per cubic foot (lb/ft³), pounds per cubic yard (lb/yd³), pounds per cubic inch (lb/in³), ounces per cubic inch (oz/in³), pounds per gallon (for U.S. or imperial gallons) (lb/gal), pounds per U.S. bushel (lb/bu), in some engineering calculations slugs per cubic foot, and other less common units.

The maximum density of pure water at a pressure of one standard atmosphere is 999.972 kg/m³; this occurs at a temperature of about 3.98 °C (277.13 K).

From 1901 to 1964, a litre was defined as exactly the volume of 1 kg of water at maximum density, and the maximum density of pure water was 1.000 000 kg/L (now 0.999 972 kg/L). However, while that definition of the litre was in effect, just as it is now, the maximum density of pure water was 0.999 972 kg/dm3. During that period students had to learn the esoteric fact that a cubic centimetre and a millilitre were slightly different volumes, with 1 mL = 1.000 028 cm3. (often stated as 1.000 027 cm3 in earlier literature).

## Measurement of density

A common device for measuring fluid density is a pycnometer. A device for measuring absolute density of a solid is a gas pycnometer .

## Density of substances

Perhaps the highest density known is reached in neutron star matter (see neutronium). The singularity at the centre of a black hole, according to general relativity, does not have any volume, so its density is undefined.

The densest naturally occurring substance on Earth is Iridium, at about is 22,650 kg/m3.

A table of densities of various substances:

 Substance Density in kg/m3 Iridium 22,650 Osmium 22,610 Platinum 21,450 Gold 19,300 Uranium 19,050 Mercury 13,580 Palladium 12,023 Lead 11,340 Silver 10,490 Copper 8,920 Iron 7,870 Tin 7,310 Titanium 4,507 Diamond 3,500 Aluminium 2,700 Magnesium 1,740 Seawater 1,025 Water 1,000 Ethyl alcohol 790 Gasoline 730 Aerogel 3 any gas 0.0446 times the average molecular mass, hence between 0.09 and ca. 10 (at standard temperature and pressure) For example air 1.2

Note the low density of aluminium compared to most other metals. For this reason, aircraft are made of aluminium. Also note that air has a nonzero, albeit small, density. Aerogel is the world's lightest solid.

Table - density of air ρ, Speed of sound in air c,
acoustic impedance Z vs. temperature °C

 Impact of temperature °C c in m/s ρ in kg/m³ Z in N·s/m3 - 10 325.4 1.341 436.5 - 5 328.5 1.316 432.4 0 331.5 1.293 428.3 + 5 334.5 1.269 424.5 + 10 337.5 1.247 420.7 + 15 340.5 1.225 417.0 + 20 343.4 1.204 413.5 + 25 346.3 1.184 410.0 + 30 349.2 1.164 406.6